Discharge lamps that utilize the phenomenon of discharge occurring within an arc tube are classified into two types: high-intensity discharge lamps and fluorescent lamps. High-intensity discharge lamps have high lamp efficacy, produce bright light, have long life, and are, therefore, highly economical lamps. Because of these advantages, high-intensity discharge lamps are widely used in outdoor lighting applications which require bright illumination over a large area.
Of such high-intensity discharge lamps, the lamp that has the highest lamp efficacy is the low-pressure sodium lamp. Low-pressure sodium lamps are therefore used in places where economy is of importance, typical applications including tunnel illumination. However, since low-pressure sodium lamps are lamps that utilize discharge in a sodium vapor, they produce monochromatic orange-yellow light near 590 nm. The result is that colors of objects illuminated by low-pressure sodium lamps are hardly recognizable.
Because of the monochromatic radiation, the low-pressure sodium lamp has had a number of problems; for example, in a tunnel, it is difficult to discern whether the color of lane-dividing lines pained on the road is white or yellow, leaving drivers unable to determine whether changing lanes is permitted or not, or almost all objects appear lacking in color and unnatural to viewers.
On the other hand, of discharge lamps, the fluorescent lamp has many advantages over other types of lamp, such as ease of lighting, excellent color rendering property, long life, and an abundant selection of light colors, and large numbers of fluorescent lamps are used in a variety of fields.
Of various types of fluorescent lamps, three band fluorescent lamps, among others, have come into wide use in recent years. The three band type fluorescent lamp produces light predominantly in three wavelength regions where the human eye is most sensitive to color perception, that is, blue at about 450 nm, green at about 540 nm, and red at about 610 nm, and thus provides good color rendering property without sacrificing brightness.
With the widespread use of the three band fluorescent lamp, one improvement after another have been made to three narrow band radiation phosphors used in the three band type fluorescent lamp. Consequently, these phosphors have excellent characteristics, such as high quantum efficiency, compared with other phosphors. Of the three narrow band radiation phosphors, the mono-phosphor green fluorescent lamp using a green phosphor expressed by the chemical formula LaPO.sub.4 : Ce.sup.3+,Tb.sup.3+, among others, has a lamp efficacy as high as about 140 lm/W in high frequency operating; its overall efficacy including the lighting circuit efficiency of lighting fixture, that is, its luminous efficacy including gear losses is about 130 lm/W. Of all the present fluorescent lamps, this fluorescent lamp has the highest luminous efficacy including gear losses. This has raised the potential for developing fluorescent lamps having high efficacy.